Major portion of the three-phase 400 Hz power in a typical aircraft system is converted into isolated, Low Voltage 28 VDC power, using simple and reliable method based on 12-Pulse Rectification. Typical converter used for that purpose, Transformer Rectifier Unit (TRU), is shown in FIG. 1. As illustrated in the figure, TRU 10 includes isolating transformer 12 having secondary windings arranged in Delta/Star, other secondary winding configurations, for example zig-zag, are being used as well. Transformer 12 is very bulky and heavy because, among other things, its primary and secondary windings must be rated for full power. Also included in TRU 10 are two 6-pulse, line-commutated rectifiers 14 and 16. TRU 10 further requires interphase reactor 18 to combine the two rectification channels into a single output, as shown in FIG. 1.
Major drawbacks of conventional Transformer Rectifier Units are:    1. Lack of voltage regulation: the output voltage fluctuates typically between 30V and 26V depending on the input voltage and load variations.    2. Transparency to voltage transients generated in the AC distribution network: the relative magnitude of the switching transients, voltage sags and surges appear on the DC side with little or no attenuation.    3. High ripple content: typically the ripple peak-to-peak voltage is in the range of 2.0 to 3.0V
In order to mitigate these undesirable effects, special pre-regulators or line conditioners are being used to protect some sensitive avionic devices and provide a better quality, regulated DC power.
The use of variable frequency power in modern aircraft systems—a radical departure from the established use of 400 Hz power—had aggravated the operating conditions for conventional Transformer Rectifier Units even further, as most of them are not designed to operate from variable frequency sources.
In airborne applications, equipment low weight is an important directive influencing a number of design decisions. For that reason, most of the prior art solutions are mainly focused on regulation aspects, but do not provide significant improvements in ripple content or immunity to input line transients. Also, the prior art devices do not include operation over the extended frequency range.
Prior art according to U.S. Pat. No. 5,541,830 achieves regulation by operating the Neutral Point Controller interconnecting either primary or secondary windings of the 12-pulse transformer. This technique, relying on classical, line-commutated conversion and relatively heavy low-frequency magnetics, has inherently slow response to rapid changes in load current and line voltage that may create instability in some systems. In addition, the ripple content in the output waveform is increased due to delayed conduction of the SCR switches. Additional filtering using heavy, low-frequency magnetics or more complex, active filtering will generally be required to achieve compliance with power quality standards.
Another prior art solution that can be used to achieve regulation in 12-pulse AC to DC conversion is shown in U.S. Pat. No. 4,488,211. Regulation is executed here by two 6-pulse SCR bridges, connected respectively to delta and wye secondary windings of the 12-pulse transformer. The conversion circuitry includes also an SCR-based active filter for suppression of harmonic and ripple effects. Like in the previous example, this solution relies on line-commutated conversion techniques to achieve regulation and uses low-frequency magnetics for isolation and filtering, leading to increase in weight and size.
The solution described in U.S. Pat. No. 4,739,466 uses a switch-mode converter cascaded with the conventional 12-pulse transformer rectifier and acting to boost the output voltage by connecting its output in series with the output of the transformer rectifier. In addition to the low-frequency components of the transformer rectifier, the circuitry includes a Boost Isolating Transformer, Boost inductor, and additional capacitor. An added weight comes from the need to increase the power rating of the 12-pulse transformer and interphase reactor in order to carry the losses in the boost section. The circuitry can operate with relatively low ripple content and immunity to the line transients due to the filtering effect the boost converter provides. Its use is limited, however, to applications where a lower efficiency can be tolerated.
Prior art according to U.S. Pat. No. 6,256,213 uses two low-frequency 12-pulse transformer-rectifiers in series-parallel connection, where the first transformer-rectifier is connected directly to the output of the converter, while the second transformer-rectifier supplies an incremental power to this output via a Boost Converter operated in a closed-loop voltage-regulation mode. The second transformer-rectifier includes an additional 12-pulse rectifier, connected in parallel to the converter's output with the purpose to support the converter's current during short-circuit and startup conditions (the boost converter inherently can not operate in a short-circuit conditions). Since the described converter provides the output power mainly through the uncontrolled rectifier bridges, it cannot disable its operation in response to abnormal conditions like overcurrent, overvoltage or overtermperature. In such cases, external means are required to disconnect the flow of power.
Common characteristic of the prior art is the use of low-frequency 12-pulse transformers for voltage reduction and isolation. The regulation techniques used in the prior art provide acceptable steady-state regulation with limited dynamic performance, inherent in circuits based on line-commutated power conversion. The output voltage typically carries a high ripple component and disturbances transpiring from the primary power lines.
A need, therefore, exists for a system, device and method to address and remedy the above-described disadvantages of the prior art.